“O Allah! I have done great injustice to myself and none except You forgives sins, so bestow on me a forgiveness from You, and Have Mercy on me, You are Often Forgiving, the Merciful”

The Hadeeth :

Abu Bakr As-Siddiq (May Allah be pleased with him) reported: I requested the Messenger of Allah (sallahu alayhi wa salam) to teach me a supplication which I could recite in my Salah (prayer). Thereupon He said, “Recite: O Allah! I have considerably wronged myself. There is none to forgive the sins but You. So grant me pardon and have mercy on me. You are the Most Forgiving, the Most Compassionate).”’

[Al-Bukhari and Muslim].

A Side Note: It is recommended to recite this du’a in the Salah after Tashahhud and before Taslim. One can also recite it at all other times.